1. Field of the Invention
This invention relates to a digital filter and moreparticularly to a digital filter to be used for a digital audio device, for example.
2. Description of the Related Art
Along with recent development in LSI, techniques for digitally processing signals have been remarkably advanced in a variety of fields such as image processing, audio devices and instrumentation. Digital filters are replacing analog filters in these fields. Specifically, digital filters are very popular in audio devices for reproducing information from compact discs (CD) and digital-audio tapes, for example. In such audio devices, signal levels of high tones are somewhat inferior to those of low and middle tones. Therefore, a master tape which serves as an original for producing compact discs is often pre-emphasized so as to raise the signal levels of high tones beforehand, thereby effectively expanding a dynamic range. To reproduce information from a compact disc which is produced from the foregoing master tape, it is necessary to de-emphasize the level via a digital filter connected to a line amplifier of a reproducing device, thereby offsetting the pre-emphasized level.
FIG. 7 of the accompanying drawings shows one example of the pre-emphasizing and de-emphasizing characteristics of a compact disc player. Audio signals recorded on a compact disc are pre-emphasized at the rate of about +6 dB/oct in a frequency band of about 3 KHz to 10 KHz, and are uniformly pre-emphasized at the rate of 10 dB in a frequency band of more than 10 KHz. When reproducing such a compact disc, the audio signals are de-emphasized at the rate of -6 dB/oct in the frequency band of about 3 KHz to 10 KHz, and are uniformly de-emphasized at the rate of -10 dB in the frequency band of more than 10 KHz.
Usually, the digital audio signals are de-emphasized by a digital filter. It is not necessary to de-emphasize audio signals which have not been pre-emphasized. With the prior art, a control routine is executed to determine whether or not to de-emphasize the signals, as shown in a flowchart of FIG. 8. Specifically, necessity of the de-emphasizing process is checked based on a code indicating the pre-emphasized state in control codes reproduced together with the digital audio signals on the compact disc (in a step S101). When the de-emphasizing process is necessary (YES in a step S102), the reproduced signals are de-emphasized by the digital filter according to a predetermined routine (in a step S103). Otherwise, the reproduced signals will be transmitted without de-emphasis via the digital filter in a step S104.
With the prior art, the necessity of the de-emphasizing process is determined depending upon whether or not the reproduced signals are input to the digital filter. A control routine for this purpose is required as well as the routine for controlling the digital filter itself. A selector circuit for this purpose is also required, which causes an increase in software and hardware for the de-emphasizing process.